A distance measurement method using a pulse laser is, generally, directed to computing a flight time of a precision laser in such a way that a laser with a shorter pulse width is emitted to an object to be measured, and a travel start time of an emitted laser is precisely measured, and an arrival time of a laser which returns after reflection on an object, the distance of which will be measured, is preciously measured, and a travel start time is subtracted from an arrival time of a laser. The distance from a laser emitting point to an object is computed by dividing, by two, the value which is obtained by multiplying the speed of light (c=2.99792458×108 m/s) by the two-way flight time of the laser. Here, what the time difference is divided by two is to compute one-way distance since light travels two ways, namely, it travels and then reflects on the object the distance of which is to be measured.
As shown in FIG. 1, the construction of the conventional high precision event timer will be described.
In other words, the conventional high precision event timer is formed of an event time signal detector 1 and a signal processor 9. The event time signal detector 1 serves to detect a signal (measurement signal) to be measured, and serves to arrange signals before the next block processes digital signals and to convert into digital signals. The signal processor 9 serves to obtain a precision event time in accordance with the signal converted into a digital form by the previous block.
As shown in FIGS. 1 and 2, the operation of the event time signal detector 1 is as follows.
The input signal (measurement signal) 11 needed for an event time measurement is converted into a signal 12 having a constant pulse width into an input signal processor CFD (Constant Fractional Discriminator) 2 of a time signal detector 1.
The conventional CFD 2 serves to make the pulse size and width constant so that the measurement values are not affected by the change of the input signal, otherwise, the results of the measurements by the next block might be affected when the sizes of the signals of a the analog type pulse waves are not uniform. The major elements belonging to the CFD 2 are a signal delay line and a voltage comparator.
The output signal of the CFD 2 having a constant pulse width irrespective of the pulse width of the input signal 11 is inputted into an integrator 4. The short pulse signal inputted into the integrator 4 is outputted in a tooth-shaped wave 13 longer than the width of the pulse.
The major elements belonging to the analog type integrator 4 are a capacitor and a computation amplifier (OP Amp).
Since it is impossible to measure the pulse time of the input signal in a method that the number of common digital pulses is counted since the operation speed of the signal to be measured is higher than the maximum operation seed of a digital semiconductor, the integrator performs a process for extending the input signal longer by an analog method.
The output signal 13 of the integrator 4 is inputted into an analog-digital converter (ADC) 5.
The analog-digital converter 5 serves to measure the voltage value based on the time of the inputted tooth-shaped wave and to transmit a signal controller 6.
A frequency multiplier 7 serves to convert the inputted low frequency signal 15 into a high frequency signal 14 which is needed at the analog-digital converter 5.
There are a variety of diverse frequency multipliers depending on the required frequency or use purposes. A voltage controlled oscillator (VCO) is mainly used, which uses a crystal oscillator when the frequency stability is high, but the frequency is low like the high precision event timer.
The controller 6 is characterized to add to the data inputted from the analog-digital converter 5, the information about the sampling time in accordance with 1 pps (Pulse per Second), which informs time of one second unit with the aid of the frequency multiplier 7, and then to transmit to an external signal processor 9.
The signal processor 9 determines a precision event time in accordance with the value of the analog-digital converter 5 transferred from the event time signal detector 1 and a time data corresponding to the value. The precision event time is determined in accordance with an inherent characteristic value 10 of the time detector in addition to the real time information inputted from the time detector when determining the precision event time.
The reference time in the apparatus which measures time corresponds to a reference signal which is important when determining the precision of the time meter, and the reference time of the conventional precision event time meter serves to generate a high frequency signal by multiplying a time sync frequency of the time meter inputted, which frequency becomes a reference time. The time of the input signal is determined in accordance with a data digitally sampled based on the reference frequency.
The detection of the actual time is performed by a digital method which is less affected by the temperature; however the CFD 2 and the integrator 4 and the frequency multiplier 7 process the signals by the analog method which is sensitive to the changes of temperature during the signal process, so the result values change sensitively to the changes of temperature. In order to overcome the above problems, the conventional high precision event timer is directed to installing a temperature control heater 3, 8 at the CFD 2 and the integrator 4 and the frequency multiplier 7, respectively, thus keeping the changes of the temperature small.
The conventional high precision event timer with the above construction needs a temperature control heater 3, 8 in order to improve the operation characteristics with respect to the temperatures of parts; however since the elements belonging to the CFD 2 and the integrator 4 both determining the precision of time in the precision event timer and the elements belonging to the frequency multiplier 7 are different from each other and have different characteristics with respect to the temperature, their characteristics change differently depending on the small changes of temperatures, so the measured result values change depending on the surrounding temperature.
In other words, in case of the capacitor which is a major element of the integrator 4, capacitance might change depending on temperature, and in case of the frequency multiplier 7, the frequency or phase of the signal might change. Since the waves are formed by the CFD 2 using a delay line the volume of which is larger than electronic elements, there are a lot of problems in manufacturing a small size high precision event timer. The characteristics of delay lines are different depending on parts since the delay lines are made manually, which results in different inherent characteristics at each time signal detector which is made from the above problematic parts. Since the signal processor 9, which generally determines precision event time, determines precision event time based on the result value of the analog-digital converter 5 having time information and the inherent characteristics of the detector which are determined during the manufacture of the time signal detector 1, it is needed to form the signal processor 9 by using a personal computer PC with an ability of computing complicated processes.